In recent years, thin films of certain species of metal chalcogenides, such as titanium sulfide (TiS.sub.2) and other transition metal sulfide materials have been studied. Films of TiS.sub.2 may be formed using chemical vapor deposition (CVD) methods, by sulfurization of titanium metal at elevated temperatures, and by sputtering methods.
In order to prepare TiS.sub.2 thin films by plasma chemical vapor deposition methods (PCVD), a titanium source, such as titanium tetrachloride (TiCl.sub.4) or Ti metal, is reacted with hydrogen sulfide (H.sub.2 S) in a plasma between 300.degree. and 450.degree. C. This results in either powder formation or deposition of a thin film, depending on the conditions. As exemplified by Kikkawa et al (Appl. Phys. A 1989, 49, 105) gaseous TiCl.sub.4 and H.sub.2 S were reacted for 10-60 minutes in a low pressure glow discharge at temperatures below 450.degree. C. to yield a thin film of TiS.sub.2. However, such techniques require that relatively cumbersome, expensive apparatus be used to generate the exacting low pressure experimental conditions.
In order to fabricate TiS.sub.2 films by low pressure chemical vapor deposition (LPCVD), gaseous TiCl.sub.4 is reacted with H.sub.2 S in a nitrogen or argon stream at low pressures (.ltoreq.30 torr). Representative of LPCVD techniques is Kanehori et al (J. Electrochem. Soc. 1989, 136, 1265), in which TiCl.sub.4 and H.sub.2 S were reacted in the gas phase at 510.degree. C. to produce stoichiometric films of TiS.sub.2. However, a major drawback of such techniques is that the deposition rates are quite slow. For example, deposition rates may vary between 3 and 9 microns per hour, depending upon the carrier gas flow rate.
A study of the formation of TiS.sub.2 films by atmospheric pressure chemical vapor deposition (APCVD) was reported by Motojima et al (Bull. Chem. Soc. Jpn. 1978, 51, 3240), in which a gaseous mixture of TiCl.sub.4 and H.sub.2 S in argon yielded TiS.sub.2 thin films at temperatures between 400.degree. and 850.degree. C. However, the crystallinity, stoichiometry and resultant film density varied markedly with temperature and flow rate. As a result, this technique makes it difficult to reproduce thin films of given characteristics with any reliability.
In such prior art methods, various problems are encountered. Relatively complex equipment and instrumentation are required to prepare the films. Additionally, use of a toxic and extremely odiferous gas, such as H.sub.2 S, is necessary. Also, the TiS.sub.2 film stoichiometry can vary significantly from the desired titanium to sulfur ratio of 1:2. Further, the film deposition rates of the prior art methods tend to be quite slow, especially where thin films of high quality are desired. Moreover, the density of the resultant TiS.sub.2 films is likely to be less than that of bulk TiS.sub.2. This may consequently lead to inferior electrical, optical and diffusion properties.
Accordingly, it would be desirable to develop a method for the production of thin films of metal chalcogenides that uses straightforward, low cost instrumentation and equipment, has high film deposition rates, utilizes less toxic and less volatile chalcogen sources, produces stoichiometric films, and affords highly dense films that exhibit superior electrical, optical, and diffusion properties.